Why man in prison for doctor's murder says he changed his testimony at trial: Part 9

Shepard said he didn't mean to kill Sonnier: "I have nothing against Dr. Sonnier or his family, or Richelle Shetina. I have nothing against any of them."
6:34 | 03/09/19

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Transcript for Why man in prison for doctor's murder says he changed his testimony at trial: Part 9
So, you can get a better look at a better price. After David Shepard's flip-flip in court, everybody wanted to know why. Why did he change his story? I went to a maximum-security prison in west Texas, where Dave Shepard was serving a life sentence, and I sat down face to face with him. This is a mountain of a man. I mean, he's got to be 6'7", 6'8", maybe 300 pounds. He is intimidating. How're you doing David? I put myself in a bad position, and I've got no one to blame but myself, and I don't blame anyone but myself. I asked him point blank, right off the bat, what happened inside that house? His claim, it wasn't a cold-blooded killing. It was a tragic accident. I'm in his backyard. He knocks on the window, and I waved, I waved at him. I said, "I'm showing you some stuff." He goes, "Oh, all right." I lift stuff up, and then the world went -- things got out of hand. Unintentionally, gun fired. I'm leaning in to see if he's okay. I fall in the window, and then, he's getting up, and going around. I'm running around to see what's going on. I said, "Dr. Sonnier, are you alright?" I don't hear anything. Go in the garage. He's laying prone. I hear his last breath. I reach down. There's no pulse. Did you stab him? I did stab him a couple of times. I can't reconcile the number that the prosecution says. Eleven, I think they've said. I had Dr. Sonnier's phone in my hand, and I considered dialing 911 just to call the authorities, and I didn't. And I put it back in. Mutilated his bodily remains, and tried to cover it and make it up look like something else. Do you consider yourself a cold-blooded killer? No, absolutely not. An idiot, you know? Careless. I mean but, no. No. Remember, in David shepherd's interview with police, he told them that when he notified Dr. Dixon that he in fact carried out the murder of Dr. Sonnier, Dr. Dixon was very pleased to hear the news. He was happy about it. He was fine with it. I said, "Mike, things didn't go how I'd hoped." And he goes, "What are you talking about?" And then I broke the news to him. And he's like "So are you telling me" -- I said, "Yeah." I said, "Mike, I'm just telling you what happened." I said, "Almost called the police. I should have, and I didn't. I'm thinking now I should have, and I didn't." And he goes, "We'll just have to see what happens." He goes, "I don't know. I don't know." But why wouldn't he have shared all that with police, who were then coming by his house talking to him? Hey, he didn't even mention your name. You'll have to ask -- well, probably to keep me out of that situation so they wouldn't even know about me. I don't know, you'll have to ask Dr. Dixon what his thinking is. But what Shepard had told me was totally at odds with what he told police. He told police that Dixon told him, "I want you to kill Sonnier." He wanted him hurt, and then it gradually developed into having him killed. Did Mike Dixon ever ask you to kill Dr. Joseph Sonnier? No, absolutely not. Absolutely not. When the police interrogated you, you said that Mike Dixon was a part of this. Right. Why did you tell them that? I embellished. I felt that's what I had to do because I already had an agreement with Mr. Matt Powell, who was the D.A., to get rid of the death penalty. So in your mind, you made up the story that Mike Dixon was involved to get the death penalty off the table. Yeah. Shepard said that Mike Dixon told him just to keep tabs on Richelle and Joseph, just to take pictures, follow him around, and see what they were up to. Why'd you help him with this? Seems kind of silly, looking back. I didn't want to offend anybody. I don't want to hurt anybody, and I sometimes have a hard time saying no, I guess. There were all these text messages that Mike Dixon was sending to you. "Go get them, whip and spur." So those text messages were referring to -- I would assume, staying on target, so you can get a picture of him with another girl besides Richelle. I didn't mean to kill Dr. Sonnier. I have nothing against Dr. Sonnier or his family, or Richelle shetina. I have nothing against any of them. I really don't. What would you say to them right now if they were watching? I'd tell them that I'm very sorry. I took your loved one. You get on the stand. You see Mike Dixon there. You see your family there. You change your story. What was that moment like for you? It was antagonizing. It was very hard. Your daughters told us that they were so upset, that this was your chance to get up there and to tell the truth, and that in their mind you lied. How does that make you feel? Pretty bad. They know me. They should know when I'm telling the truth and when I'm not. And I was telling the truth on the stand. Do you miss them? Oh, every day. I'm sorry. The damage is done. There's not anything that's going to repair our relationship. There's nothing that can fix it, or make it right even if he were to get out. Do you think your dad should spend the rest of his life in prison? Yes. Absolutely, yes. We have absolutely, you know, accepted it, and proven that we can move on, and we can be better, and we can absolutely overcome it, and be stronger because of it. Despite what happened with David Shepard's extraordinary turnaround, the prosecution was not about to give up. One year later, they're back in court, ready to try Dr. Dixon one more time. Matt Powell, without blinking, he says, "We are going to get this guy. We are going to do this again. Don't you worry. I've got you." And that's all I needed to hear. Amarillo Dr. Michael Dixon is back on trial for capital murder. When the witness list comes out for the second trial, David Shepard's name is not to be found, but on the list is his daughter Haley. The number one most important thing from this second trial was the unbelievably courageous testimony of Haley Shepard.

This transcript has been automatically generated and may not be 100% accurate.

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